When I used to tank this on my guardian, his last invulnerability phase would sometimes not end until I was on top of him. Not sure how/why that triggers (or fixes the trigger for) his vulnerability, but give that a try next time. You should have no trouble closing to him even at slow-walk, as long as you pop cds and/or your healer stays on the job.
I have a feeling that this might be the case. I have never seen this bug happen before but I always rush towards him and close the gap. I do that even when I'm playing sentinel; pop Transcendence, leeroy in and start bashing on him while the rdps takes care of the weak adds.